Due to their high efficiency and fuel flexibility and their compatibility with inexpensive interconnect materials, intermediate temperature fuel cells are attractive alternatives to combustion engines for the conversion of chemical to electrical energy. Amongst fuel cell types suitable for intermediate temperature operation (100-300° C.) solid acid fuel cells (SAFCs) offer the unique benefit of a truly solid electrolyte, specifically, CsH2PO4, that, in turn, provides significant system simplifications relative to phosphoric acid or alkaline fuel cells. Despite these benefits, however, the power output of SAFCs have not yet reached levels typical of conventional polymer electrolyte or solid oxide fuel cells. In general, successes in reducing electrolyte losses, through fabrication of thin-membrane fuel cells, have outpaced successes in reducing electrode losses. While SAFCs with electrolyte thicknesses of 25-50 μm can be routinely fabricated, Pt loadings remain as high as ˜4 mg/cm2, and even at this level, the electrodes are responsible for the majority of the overpotential losses.
Because the components in typical SAFC electodes, Pt, CsH2PO4 and pores, can each transport only one species (electrons, protons and gas phase molecules, respectively), one can surmise that the electrocatalysis reaction is limited to the triple phase boundaries at which the electrolyte, catalyst and the gas phase are in contact and where the simultaneous and coordinated transport of electrons, ions, and gas molecules can occur. Maximization of the triple-phase boundary per unit projected area is thus a recurring theme in composite electrode systems. In the case of SAFC electrodes, the possibility of attaining a high density of TPBs as implied through the use of nanoparticle Pt has not been realized because of the typically large size of the electrolyte particles. Recent successes in incorporating submicron CsH2PO4 in SAFCs suggest that intimate mixing of nanosized particles of the two components (electrolyte and catalyst) would dramatically enhance the contact area between the two phases. A composite electrode must further ensure continuous pathways for ion, electron and gas phase transport. Accordingly, there is a need to to fabricate an interconnected, porous, three-dimensional nanostructured composite. Such a structure is expected to provide the dual benefits of enhanced electrochemical activity and reduced Pt loading.
The fabrication methodology utilized here is the electrospray technique, sometimes termed electrostatic spray deposition (ESD). The process relies on electrostatic forces to expel micrometer sized droplets from a charged liquid. The liquid is pumped through a capillary, and, under ideal conditions, the applied electric field causes the liquid to emerge in the shape of a cone, called the Taylor cone. The high electric field concentrated at the tip of the cone induces the emission of a fine spray of charged droplets. The electrospray method has been widely used to aerosolize liquids, and is particularly useful in the study of macromolecules. As a fabrication tool, the method has been largely limited to the preparation of polymeric nanofibers, and occasionally the deposition of thin films (where examples exist for polymers, ceramics and metals, although most studies involve polymers). The potential of ESD for the preparation of nanostructured, highly porous films of non-polymeric materials has been largely overlooked. Surprisingly, the present invention meets the need for electrode fabrication, as well as other needs, using the ESD method.
The present invention relies on the recognition that, if the liquid used for ESD is a solution of solvent and solute and sufficient evaporation occurs as the droplets are accelerated towards the grounded substrate, the resulting charge concentration induces break-up of the droplet and the ultimate deposition of sub-micron to nanoscale, solvent-free particles on the substrate. A wide range of chemical and physical parameters can be varied to tune the characteristics of the resultant structure which can span from dense thin films to porous electrodes. These parameters include solvent concentration, solution composition (affecting solution conductivity, surface tension, viscosity), spraying temperature, gas flow rate, and spray geometry (e.g. tip-to-substrate path length, spraying direction).